The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the ossicular chain, excessive cerumen, or a malformed Tympanic Membrane. Conductive hearing losses may be treated with acoustic hearing aids, or a bone conduction hearing aid.
Sensorineural hearing loss, on the other hand, is primarily caused by the absence or destruction of the hair cells on the basilar membrane. To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the major part of the ear by presenting electrical stimulation directly to the auditory nerve fibers by way of one or more channels formed by an array of electrodes implanted in the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function. Cochlear implants are typically capable of providing information in higher frequencies such as up to 8 kHz or even higher.
There is a group of people that has some degree of residual hearing, which may also include moderate to severe hearing loss, typically in the low frequencies (e.g., below 1 kHz) and a severe-to-profound hearing loss usually in the high frequencies (e.g., above 1 kHz). However, this group may also include people who retain residual hearing in non-contiguous frequency ranges such as below 1 KHz but also between 2 and 3 kHz and so on.
These people having residual hearing cannot benefit from traditional amplification because of the severity of the hearing loss generally in the high frequencies. Nor are they classic cochlear implant candidates, because of their mostly intact residual hearing.
For this group of people, various dual-mode stimulators such as electro-acoustic stimulation (“EAS”) systems have been developed that provide such patients with the ability to perceive sound both in frequency ranges where residual hearing is retained, i.e. residual frequency range and frequency ranges where residual hearing is absent, i.e. non residual frequency range. Electro-acoustic stimulation refers to the use of an acoustic hearing aid and a cochlear implant together in the same ear. The acoustic hearing aid acoustically amplifies the signal in residual frequency range, i.e. frequencies where residual hearing is retained typically in low frequencies, while the cochlear implant electrically stimulates in the non-residual frequency range, i.e. frequencies where residual hearing is not retained or absent typically because of loss or destruction of hair cells. The auditory nerve combines the acoustic and electric stimuli to one auditory signal. Results of various studies have shown a highly synergistic effect between hearing aid and cochlear implant technology, particularly evident in speech understanding, pitch discrimination, and music appreciation.
However, electro-acoustic stimulation systems suffer in performance because the perceptual sensitivity of a patient to acoustic stimulation is quite different in absence of electrical stimulation vis-a-vis when electrical stimulation by way of cochlear implant comprising an implantable electrode array is present in cochlea of the patient. Unfortunately, currently available solutions do not address this issue effectively, thereby making it difficult for a patient to adjust to such dual-mode stimulation and a patient unknowingly suffers from sub-optimal EAS system performance. Some solutions recommend partial insertion or short electrodes in order to avoid effects on cochlear mechanics in the residual frequency range. However, this has a significant drawback that electrical stimulation in the residual frequency range is not possible if a part of the residual hearing is lost, that is residual frequency range is narrowed for reasons such as consequence of the cochlear implant surgery or over time due to aging.
There exists a need to offer an alternative to currently available solutions.